The invention pertains to strobe lights driven by programmed processors. More particularly, the invention pertains to such strobes which respond to variable input voltages and wherein in-rush currents are limited.
Circuits for driving strobe lights of a type usable in alarm systems are known. Some known circuits charge a capacitor using constant frequency, variable current signals. Others have incorporated a coil in combination with frequency varying circuits. One known system has been disclosed in U.S. Pat. No. 5,850,178, issued Dec. 15, 1998, entitled xe2x80x9cSynch Module With Pulse Width Modulationxe2x80x9d and assigned to the assignee hereof.
Known circuits have been designed to be driven from a single nominal voltage such as 12 volts or 24 volts. In addition, known circuits have been designed to drive a gas filled tube to produce a single, nominal candela output.
There is a need for more flexible strobe drive circuitry. Preferably a single drive circuit could accommodate a range of nominal input voltages. In addition, it would be desirable to be able to select from a range of desirable candela output levels without regard to available input voltage.
Finally, it would be preferable if in-rush currents could be limited under various conditions. One known system is disclosed in Ha et al U.S. Pat. No. 6,049,446 assigned to the assignee hereof and entitled, xe2x80x9cAlarm Systems and Devices Incorporating Current Limiting Circuitxe2x80x9d, incorporated herein by reference.
Preferably, the above noted features could be implemented so as to promote manufacturability, and to limit operating in-rush currents. It would also be preferable if such flexibility did not appreciably increase unit cost.
A strobe drive circuit combines circuits to accept variable input drive voltages with circuitry responsive to selectable candela output levels. In one aspect, the circuitry monitors the time to charge a capacitor to a selected, predetermined voltage. In another aspect, the actual capacitor voltage is monitored. A gas filled tube can be triggered at the appropriate voltage. Other types of visible output devices could also be used.
The charging duty cycle can be varied to respond to various input voltages as well as differing predetermined flash voltages. The duty cycle of the drive current is continually corrected with each flash.
In one embodiment, surge currents are substantially eliminated by starting with a lower duty cycle and increasing same over time, with each flash. With this configuration, power supply fold back or over-current conditions can be substantially eliminated.
In another aspect, the charging current duty cycle can be incremented one or more times from an initial value while charging the capacitor. Simultaneously, the capacitor""s voltage can be monitored. Depending on the results, for example the value of the flash voltage of the present flash cycle, the current charging current duty cycle can be altered for the next flash cycle.
In another aspect, a current smoothing circuit limits initial turn-on current for a predetermined interval after power is applied. For example, turn-on current can be limited for an interval in a range of 300-700 ms with 500 ms being a preferred interval. This is especially advantageous where numerous strobes are connected to a common power source.
Where synchronization pulses are applied to the drive circuit, for example from an external source which might be a fire alarm system, capacitor charging can be interrupted or terminated when such pulses are present. This will minimize charge depletion from the capacitor(s).
Where applied energy is in the form of full wave rectified AC, surge currents can be minimized after each flash by commencing charging (after each flash) by waiting till the rectified AC voltage drops to a predetermined low value. For example, charging can be commenced once the applied AC drops to about zero volts.
A programmed processor can be incorporated into the control circuitry. Information can be stored relative to a plurality of available candela outputs. When a specific output has been selected, corresponding pre-stored information is used by the processor to charge the capacitor to the respective output voltage.
In another embodiment, the capacitor voltage can be measured, digitized in an A/D converter, and compared to a plurality of pre-stored values. In response to the comparison step, charging current duty cycle can be altered.
The control process also responds to input voltage variations. With a lower input voltage, the charge current duty cycle will increase to provide the necessary capacitor voltage. With a larger input voltage, the duty cycle will decrease.
A control method includes the steps of establishing a plurality of target pulse widths based on respective candela outputs; selecting a candela output level; charging an energy source until either a selected voltage is reached or until a predetermined time interval has ended; keeping track of the actual charging time interval; comparing the actual charging time interval to the target pulse width associated with the selected candela output; where the actual time interval is less than the target pulse width, decreasing the charging parameter a selected amount and where the actual time interval is greater than the target pulse width, increasing the charging parameter.
Where the selected voltage is repetitively reached before the predetermined time interval has ended, the charging parameter can be repetitively reduced. This reduction can be via a decreasing amount. Where the predetermined time interval repetitively ends before the selected voltage has been reached, the charging parameter can be repetitively increased.
In another embodiment, capacitor voltage can be digitized and compared to a candela specific target value. Depending on the results of this comparison, charging duty cycle can be altered.
In either embodiment, the closed loop control system responds to variations in input voltage. Charging duty cycle is adjusted in response thereto to maintain a selected candela output level. Variations in the input voltage in a range on the order of 4:1 can be accommodated.
Desired candela output level can be manually set at a unit. Alternately, it can be downloaded to a unit, as a programmable parameter, from a remote source.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.